Why isn't a button added in the panel by running this script?
Line 16 should read:

layout.operator("pose.select_all", text="Legs.Idle.1", icon="MOD_ARMATURE")


You aren't giving the full path and the parameters. You are choosing an operator so Blender already knows that it is in "bpy.ops". When the user pushes the button, he will set the values.

If you want to make sure you use your own hard-coded values, make your own operator class, that calls the bpy.ops.pose.select_all() operator inside of it with desired parameters, or catch the result of the operator layout definition in a variable, and set the parameters like this:

# catch the defined operator in a variable
myOperator = layout.operator("pose.select_all", text="Legs.Idle.1", icon="MOD_ARMATURE")

# set the options on that var. Add as many as needed
myOperator.action='TOGGLE'